Singlet provides threadsafe, generic singletons for Golang.

The common pattern of using sync.Once to implement a global singleton doesn't work well when the singleton value includes a generic type, since type arguments may not be available at a global level. Singlet provides a solution, allowing you to create threadsafe, generic singletons, anywhere in your code.

To use singlet, first create a Singleton. Then call GetOrDo which will create and store a value in the Singleton, if one doesn't already exist, by calling the provided func, else it will return the existing value:

var s = &singlet.Singleton{}
cache1, _ := singlet.GetOrDo(s, cache.New[int]) 
cache2, _ := singlet.GetOrDo(s, cache.New[int]) // cache.New is only called once

if cache1 != cache2 {
    panic("caches should be equal")
}

You can also get a previously created value for a Singleton:

cache, _ := singlet.Get[*Cache[int]](singleton)

Calling Get or GetOrDo for a result type that doesn't match the previously stored result type for a Singleton will result in an ErrTypeMismatch:

singlet.GetOrDo(s, cache.New[int])
singlet.Get[string](s) // Returns ErrTypeMismatch

Copyright Jonathan Halterman. Released under the Apache 2.0 license.